Microwave waveguide water load employing a quarter wave window of reduced characteristic impedance



Dec. 1, 1970 N. H. WILLIAMS 3,544,

MICROWAVE WAVEGUIDE WATER LOAD EMPLOYING A QUARTER WAVE 'II'LNUOW OF REDUCED CHAIIACTERISTIC IMPEDANCE Original Filed Nov. 10, 1966 FIG.I FIG. 2 PRIOR ARI I 8 5 PRIOR ARI Z Z T w c 9 '2 5 I B M K 1 AIR WATER 1* j I /l lA iB. 4 ll 49:1 I 5 T CERAMIC 2 TEFLON a Hfi- T M 7 WATER i AIR |2 4 WATER Io 7 l [:9 TEFLON FIG 3 INVENTOR.

NORMAN H. WILLIAMS BY Mfi ATTORNEY United States Patent 3,544,923 MICROWAVE WAVEGUIDE WATER LOAD EM- PLOYING A QUARTER WAVE WINDOW 0F RE- DUCED CHARACTERISTIC IMPEDANCE Norman H. Williams, San Francisco, Calif., assignor to Varian Associates, Palo Alto, Calif., a corporation of California Continuation of application Ser. No. 593,423, Nov. 10, 1966. This application Oct. 30, 1969, Ser. No. 871,776 Int. Cl. H01p 1/26 U.S. Cl. 333-22 6 Claims ABSTRACT OF THE DISCLOSURE An inexpensive waveguide type water load for the absorption of microwave energy which utilizes a window material having a dielectric constant lower than that of alumina is disclosed together with the design criteria and structural requirements to enable such a water load to provide a proper impedance match over a useful bandwidth.

This application is a continuation of application Ser. No. 593,423, filed Nov. 10, 1966.

The present invention relates in general to microwave waveguide water loads and, more particularly, to an improved load employing a microwave window structure forming a quarter wave transformer section having a lower characteristic impedance, than used heretofore, to obtain an impedance match between the air filled input guide and the liquid dielectric filled loss section of the guide, whereby window materials having a lower dielectric constant than alumina, such as Teflon, may be employed while obtaining operating bandwidth of approximately 7%.

Heretofore, microwave waveguide water loads have been built wherein an alumina microwave window formed a quarter wave transformer section between an air filled waveguide section and a water filled waveguide section. The water filled waveguide section formed the loss section of the water load. Such a water load is described and claimed in copending US. application Ser. No. 474,414, filed July 23, 1965, now issued as U.S. Pat. 3,360,750 and assigned to the same assignee as the present invention.

In this prior art, it was taught that the dielectric constant E of the quarter wave window should be equal to /E E where E; is the dielectric constant of the air filled section of guide and Eg is the dielectric constant of the liquid dielectric filled loss section of guide and E is typically 77 for water at 60 C. Thus, alumina with a dielectric constant of 9 closely approximates the proper window material for a quarter wave window member, assuming the air filled, window filled and water filled guide sections are all of equal transverse dimensions.

While alumina windows perform very well under the conditions encountered in high power water loads, they tend to be relatively expensive to manufacture and for certain moderately high power applications, such as for example in the tens of kilowatts average power range, it would be desirable if a less costly Teflon window member could be utilized.

However, when one attempts to design a Teflon window with a dielectric constant of 2, as contrasted with 9 for alumina, such a window must be placed in a section of 3,544,923 Patented Dec. 1, 1970 waveguide having a lower height to satisfy the quarter wave transformer relation Z /Z Z where Z is the characteristic impedance of the quarter wave section of guide with the window in place, Z is the characteristic impedance of the air filled guide, and Z is the characteristic impedance of the water filled guide. However, when the above condition is satisfied the waveguide is found to be so severely mismatched that it is essentially inoperative for practical purposes.

In the present invention, it has been found that the effective size of the liquid dielectric filled loss section of guide, to be used in the aforecited quarter wave transformer equation, is not the actual size of this section of guide, nor the size of the air filled section of guide, but is the size of the window section of guide which opens into the loss section of guide. When these smaller effective dimensions are employed for determining the characteristic guide impedance of the loss section, the resultant quarter wave window design provides a water load having substantial useful bandwidth and permits the use of window materials having dielectric constants substantially less than that of alumina. Thus, the waveguide water load of the present invention can be manufactured for less cost than the prior art water load which employed an alumina window.

The principal object of the present invention is the provision of a waveguide water load which is less expensive to manufacture.

One feature of the present invention is the provision of a waveguide water load employing a quarter wave window member made of a material having a dielectric constant substantially less than that of alumina and disposed in a window section of waveguide having a characteristic impedance Z according to the following equation i /Z Z" where Z is the characteristic impedance of the waveguide on the R.F. input side of the window, and Z is the effective characteristic impedance for the liquid dielectric filled loss section on the RP. output side of the window and assumes the guide in the loss section regardless of its larger dimensions has the same transverse dimensions as the section of guide containing the window member, whereby a broadband water load is obtained.

Another feature of the present invention is the same as the preceding feature wherein the height of the waveguide section containing the window member is substantially less than the height of the waveguide section on the R.F. input side of the window.

Another feature of the present invention is the same as any one or more of the preceding features wherein the dielectric window material is Tefion, whereby the manufacturing costs of the water load are reduced as compared with prior loads using alumina window members.

Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:

FIG. 1 is longitudinal sectional schematic line diagram of a prior art water load,

FIG. 2 is an equivalent circuit for the water load of FIG. 1,

FIG. 3 is a longitudinal sectional schematic line diagram of the water load of the present invention.

FIG. 4 is an equivalent circuit for the water load of FIG. 3,

FIG. is a longitudinal section view of a water load of the present invention,

FIG. 6 is a sectional view of the structure of FIG. 5 taken along line 66 in the direction of the arrows.

Referring now to FIG. 1 there is shown, in schematic line diagram form, the prior art water load 1. More particularly, an air filled section of rectangular waveguide 2 is provided with an input flange 3 for connecting to a section of waveguide, not shown, on a device to be loaded, such as, for example, a high power S-band microwave tube. An alumina ceramic window member 4 is sealed across the end of the input section of guide 2 to form a wave permeable liquid tight barrier. The sealed end of the guide 2 at 5 projects into a loss section of rectangular waveguide 6 having a height substantially equal to the height of the input section of guide 2 and filled with tap water. Water is fed into the loss section of guide 6 at 7 and exhausted via output ports 8 and 9.

The alumina window member 4 is dimensioned to be an odd number of quarter wavelengths long at the center of the pass band of the water load 1. With an alumina window 4, the equivalent circuit for the water load 1 is as shown in FIG. 2. The characteristic impedance of the window section of guide Z i.e., the alumina filled section of guide 2, is approximately equal to /Z Z where Z is the characteristic impedance of the air filled section of guide 2 and where Z is the characteristic impedance of the water filled loss section of guide 6. Any slight capacitive impedance mismatch at the window 4 is matched out by an inductive reflective discontinuity 11 placed one quarter of a guide wavelength in front of the window 4. Such a water load has a power handling capability in excess of tens of kilowatts C.W. at S-band over a bandwidth, between 3 db points, in excess of 13%.

Referring now to FIG. 3 there is shown a water load incorporating features of the present invention. In this embodiment, a quarter wave window member 12 having a substantially lower dielectric constant than that of alumina, such as for example Teflon, is employed.

The characteristic impedance Z of the window section of guide 13 (see FIG. 4) is determined by the equation Z /Z Z where Z is the characteristic impedance of the air filled input section of waveguide 2 and where Z is the equivalent characteristic impedance of the loss section of waveguide 6 assuming the loss section of guide is filled with the lossy dielectric liquid (water) and has transverse dimensions equal to the transverse dimensions of the window section of waveguide 13 as indicated by the dotted lines 14. When the window section 13 is dimensioned according to the above relationship, the water load 10 has a bandwidth of approximately 7% between 3 db points at S-band and will dissipate up to on the order of tens of kilowatts C.W. microwave power.

Assuming that the waveguide sections 2, 13 and 6 are all rectangular operating in the TE mode, that all three sections of guide have the same width, that the input section of guide is air filled, that the loss section of guide appears very wide such that its ratio of guide wavelength to free space wavelength (Ag/M is one and, that the effective height of the guide I1 in the loss section is equal to the height of the guide in the window section h then:

h =& (M) (if h {1Y X 0 MI 1 0 where K and K are the relative dielectric constants of the window member 12 and lossy liquid dielectric, respectively, (Ag/x) is the ratio of guide wavelength to free space wavelength in the air filled input section of guide 2, (x/Agh is the ratio of free space wavelength to the guide wave length in the window member 12, and h is the height of the input section of guide 2. For standard 4 WR-340 Waveguide at 2450 mHz., I1 is equal to 0.44 inch for a Teflon window.

Referring now to FIGS. 5 and 6 there is shown a waveguide microwave water load 21 of the present invention. A section of standard S-band rectangular waveguide 22, as of bronze, is closed at one end by a transverse cover plate 23, as of bronze. A pair of pipe fittings 24 and 25 pass through the cover plate 23 for passing liquid coolant, as of water, into and out of the closed section of guide 22. An input pipe 26, 3 inches in length, extends axially into the loss guide 22 to within about 0.75 inch of a reduced height window section of guide 27. The reduced height guide 27 is made of a conductive corrosive resistant material such as bronze.

A dielectric block window member 28, as of Teflon, is sealed across the opening in the reduced height section of guide 27 by an epoxy adhesive sealant coating the periphery of the block window 28. An iris plate 29 is soldered across the flanged end of the waveguide 22. The iris plate 29 has an iris opening 31 in registration with the block window member 28. The iris plate has a thickness of 0.125 inch and its iris opening is slightly smaller than the block window 28 to provide a mechanical retaining lip around the marginal edge of the iris to hold the window member 28 against the water pressure in the loss section 6 of the guide. The iris opening is 0.342 inch X 2.843 inch. The block window 28 is 0.450 inch x 0.950 inch x 3.40 inches. The 0.950 inch dimension is essentially a quarter guide wavelength at 2450 mHz. The waveguide 22, on the inside, is 3.40 inches by 1.70 inches and 4.75 inches long from the iris plate 29 to the cover plate 23. The flanged end of the waveguide 22 is adapted to mate with a section of air filled standard WR340 waveguide, not shown, having a height of 1.70 inches and a width of 3.40 inches.

In operation, microwave energy to be absorbed is applied to the water load 21 via an air filled input section of waveguide, not shown, coupled to the flanged end of the waveguide 22. The microwave power is coupled through the window 28 into the lossy water filled section of guide 6 and absorbed. Water is fed into the load 21 via input pipe 26 which directs its flow against the window 28. The warm liquid is exhausted via output tube 25. The load 21 absorbs 2.5 kw. of CW. power at S-band with a pass band of 7% between 1.3:1 VSWR points.

Since many changes could be made in the above construction and many apparently widely ditferent embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a microwave water load means forming a section of hollow waveguide having an input terminal to which microwave energy to be absorbed is applied, a loss section to contain a lossy dielectric liquid for absorbing the microwave energy, and a window section, means forming a block of solid dielectric material filling said window section of waveguide and having a length which is substantially an odd integral number of quarter guide wavelengths long at the center frequency of the pass band of the water load, said window section of waveguide having a characteristic impedance Z with the solid dielectric fill in place substantially equal to /Z Z where Z is the characteristic impedance of the input waveguide section adjacent said window section of waveguide, and where Z is the effective characteristic impedance of the liquid dielectric filled loss section of waveguide assuming said loss section of waveguide had transverse dimensions substantially equal to the transverse dimensions of said solid dielectric block material where at least the height of said loss section of guide is actually greater than the height dimension of said window, and wherein said solid dielectric b k wi ow material has a dielectric constant substantially less than that of alumina, whereby a window material having lower dielectric constant than that of alumina may be employed in a water load having substantial bandwidth.

2. The apparatus of claim 1 wherein the lossy liquid dielectric is water.

3. The apparatus of claim 1 wherein the height of said quarter wave window section of waveguide is substantially less than the height of the input waveguide section adjacent said window section of waveguide.

4. The apparatus of claim 1 wherein said solid dielectric window block is made of Teflon.

5. The apparatus of claim 1 wherein said window section of waveguide is one quarter of a guide wavelength long with said dielectric window member in place.

6. The apparatus of claim 1 wherein the width of said 15 window section of waveguide is substantially the same size as the actual width of said loss section of waveguide.

References Cited UNITED STATES PATENTS 10 HERMAN KARL SAALBACH, Primary Examiner M. NUSSBAUM, Assistant Examiner US. Cl. X.R. 33398 

